Reference

ValueType requirements

Values are strongly informational objects for which
identity is not significant, i.e. the focus is principally on
their state content and any behavior organized around
that. Another distinguishing feature of values is their
granularity: normally fine-grained objects representing simple
concepts in the system such as quantities.

As the emphasis of a value lies in its state not its
identity, values can be copied and typically assigned one to
another, requiring the explicit or implicit definition of a
public copy constructor and public assignment operator. Values
typically live within other scopes, i.e. within objects or
blocks, rather than on the heap. Values are therefore normally
passed around and manipulated directly as variables or through
references, but not as pointers that emphasize identity and
indirection.